/*
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package io.rsocket.internal.jctools.queues;

final class IndexedQueueSizeUtil {
  public static int size(IndexedQueue iq) {
    /*
     * It is possible for a thread to be interrupted or reschedule between the read of the producer and
     * consumer indices, therefore protection is required to ensure size is within valid range. In the
     * event of concurrent polls/offers to this method the size is OVER estimated as we read consumer
     * index BEFORE the producer index.
     */
    long after = iq.lvConsumerIndex();
    long size;
    while (true) {
      final long before = after;
      final long currentProducerIndex = iq.lvProducerIndex();
      after = iq.lvConsumerIndex();
      if (before == after) {
        size = (currentProducerIndex - after);
        break;
      }
    }
    // Long overflow is impossible (), so size is always positive. Integer overflow is possible for
    // the unbounded
    // indexed queues.
    if (size > Integer.MAX_VALUE) {
      return Integer.MAX_VALUE;
    } else {
      return (int) size;
    }
  }

  public static boolean isEmpty(IndexedQueue iq) {
    // Order matters!
    // Loading consumer before producer allows for producer increments after consumer index is read.
    // This ensures this method is conservative in it's estimate. Note that as this is an MPMC there
    // is
    // nothing we can do to make this an exact method.
    return (iq.lvConsumerIndex() == iq.lvProducerIndex());
  }

  public interface IndexedQueue {
    long lvConsumerIndex();

    long lvProducerIndex();
  }
}
